Pointing devices are computer peripherals used for drawing or navigating on a computer system. They consist of a flat sensing area and an associated pointer. They can be in the form of a graphics tablets, comprised of a sensing tablet and a pointing tool that is usually either pen shaped or mouse shaped. Graphics tablets measure X and Y coordinate positions of the tool as well as other parameters, such as pen tip pressure, pen tilt angle, pen axial rotation, mouse buttons, mouse rotation, etc. A user manipulates the tool over the tablet surface for tracing, drawing, writing, manipulating graphical data, or navigating in a graphical user interface. The tablet is usually attached to a host computer and the graphical data is displayed either on a separate display or one integral with the computer.
Another form of pointing device is a touch tablet. A touch tablet has a surface that is sensitive to being touched by a user's finger, which acts as the pointing tool. The touch tablet measures X and Y coordinate positions, contact with the user's finger, and other parameters such as contact area. The user touches the surface of the touch tablet, usually placed in front of a display, to select or manipulate objects on the display.
Various underlying technologies can be used for detecting the pointing device or tool position in graphics tablets. One common technology is electromagnetic, where the position of the tool is detected by interpolating signal strength across a series of sensing coils in the tablet. Another technology is electrostatic, where the position of the tool is detected by interpolating signal strength across a series of electrodes in the tablet. Touch tablets can use capacitive coupling between the user's finger and the tablet to detect finger position. Here, the position of the user's finger can be interpolated from signal levels at electrodes at the edges of the tablet, or between a series of electrodes located in the tablet. An electric field created by the electrodes can extend above the surface of the tablet to allow measurement of the coordinates above the surface. The detection, interpolation, and transmission of the resulting coordinate position is preformed in real-time so that the user can observe his or her actions on the computer monitor or display device.
Since the detection of the signal strength at the electrodes of the touch tablet or at the coils of the graphics tablet requires measuring the amplitude of an analog signal, the signal is susceptible to interference or noise from external sources. This noise manifests itself as instability in measured coordinate positions and is known as jitter. Jitter has a fairly random distribution usually with Gaussian properties and a predictable range for any given environment, but is unpredictable from sample to sample.
Jitter has a time scale that is the same as the time scale of the sampling, which is generally around 100-200 Hz. Since user movements are generally on a time scale of less than 20 Hz, a variety of low-pass filters can be used to separate the two signals, that is, the coordinate position signal from the noise or interference created by the external sources. However, significant effort must be taken to avoid introducing filtering artifacts. Filtering artifacts occur when the low pass filtering removes high frequency content from the position signal which actually represents valid user input data.